Integrated PM Filter and SCR Catalyst for Lean Burn Engine

ABSTRACT

A wall-flow exhaust gas emissions aftertreatment device for simultaneously reducing the particulate matter and NOx content of the exhaust. The device comprises a number of longitudinal inlet channels and outlet channels, the inlet channels being plugged at the exit face, and the outlet channels being plugged at the entry face. The interiors of the inlet channels are coated with a particulate matter catalyst coating, and the interiors of the outlet channels are coated with a selective catalyst reduction catalyst coating.

TECHNICAL FIELD OF THE INVENTION

This invention relates to reducing exhaust emissions from lean burn internal combustion engines, and more particularly to an emissions control device that integrates the functions of particulate matter and NOx reduction.

BACKGROUND OF THE INVENTION

Internal combustion engines used for both mobile and stationary applications are subject to strict emission limits. Approaches to reducing emissions include improved in-cylinder combustion designs or fuel modifications, but these improvements have fallen short of meeting emissions limits. Other approaches involve the use of exhaust aftertreatment devices, which have achieved significant emissions reductions.

For lean burn internal combustion engines, such as diesel engines, the main pollutants of concern are oxides of nitrogen (NOx) and particulate matter (PM). The latter is composed of black smoke (soot), sulfates generated by sulfur in fuel, and components of unburned fuel and oil.

To reduce NOx, one approach is the use of NOx reduction devices, such as lean NOx traps (LNTs), lean NOx catalysts (LNCs), and selective catalytic reduction (SCR) catalysts. For most heavy-duty engines, and for some medium-duty and light-duty engines, SCR catalysts are preferred over other NOx reduction devices due to their high efficiency and reduced sensitivity to lubricant oil poisoning.

To reduce PM, one approach is the use of various types of diesel particulate filters (DPFs). PM filters have been developed with catalyst material coated on the filter. These filters are known as “catalyzed particulate filters”.

NOx reduction devices and DPFs may be used alone or together, with either or both being used downstream of the engine, in the exhaust line.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:

FIG. 1 illustrates a lean burn engine system having a PM-SCR catalyst in accordance with the invention.

FIG. 2 is a side view of the PM-SCR catalyst.

FIG. 3 is a perspective view of the exit face of the PM-SCR catalyst.

FIG. 4 is a partial sectional view of the PM-SCR catalyst, illustrating the coated inlet and outlet channels.

DETAILED DESCRIPTION OF THE INVENTION

The following description is directed to an emissions aftertreatment device, for use in the exhaust system of a lean burn engine, which integrates the functions of a PM filter and SCR catalyst. It is referred to herein as a “PM-SCR catalyst”.

An advantage of the PM-SCR catalyst is that it reduces PM and NOx emissions simultaneously with a single device. It is available in a compact package, and minimizes the space requirements for effective emissions aftertreatment.

FIG. 1 illustrates a lean burn engine system, generally identified as 10, having a PM-SCR catalyst 11 in accordance with the invention. In the illustrative embodiment, system 10 has a diesel engine 12, an exhaust gas recirculation (EGR) loop 13, and is an air-boosted system having a turbocharger 26. Examples of lean burn engines other than diesel engines are gasoline direct injection (GDI) engines and some alternative-fueled engines.

The direction of flow of exhaust gas through the EGR loop is indicated by directional arrows in FIG. 1. Exhaust gas discharged from the engine's exhaust manifold 14 is directed through the EGR loop, which may include a filter and/or heat exchanger (not shown). The recirculated exhaust gas flows to an EGR valve 18, and then to the engine's intake manifold 22 where it is mixed with fresh air supplied via intake duct 24.

The engine's intake air is compressed by the turbocharger's compressor 26 a, which is mechanically driven by its turbine 26 b. Desirably, the compressed air discharged from the compressor 26 a is cooled through an intercooler 30 positioned between the compressor 26 a and the intake manifold 22.

PM-SCR catalyst 11 is located downstream of the turbocharger compressor. As explained below in connection with FIGS. 2-4, the engine's exhaust is treated by catalyst 11, which reduces both PM and NOx in the exhaust. The treated exhaust exits the catalyst 11 into the atmosphere (or into other downstream aftertreatment devices not shown).

Control unit 20 may be processor-based, programmed to control various aspects of engine operation. In general, control unit 20 may be implemented with various controller devices known or to be developed. Further, control unit 20 may be part of a more comprehensive engine control unit that controls various other engine and/or emissions devices.

FIGS. 2-4 illustrate PM-SCR catalyst 11 in further detail. As explained below, catalyst 11 is a dual-coated wall-flow device, which simultaneously reduces both PM and NOx emissions from a lean burn engine.

Referring particularly to FIGS. 3 and 4, catalyst 11 comprises a number of longitudinal walls between an entry face 31 and exit face 32. These walls define channels 201 and 202, such that the end faces of catalyst 11 form a honeycomb pattern.

Inlet channels 201 are open at the entry face and closed (plugged) at the exit face; outlet channels 202 are closed (plugged) at the entry face and open at the exit face. Exhaust enters the open ends of the inlet channels at entry face 31, and exits the open ends of the outlet channels at exit face 32.

Typically, the number of inlet channels and the number of outlet channel are substantially equal. Their respective ends are plugged in an alternating pattern, such that the entry and exit faces form a checkerboard pattern.

By “wall flow” is meant that the exhaust gas flows through the inlet channels 201 to their dead ends. PM particles are filtered by the porous walls of the inlet channels 201, and deposit themselves in these channels 201. After the exhaust passes through the walls of the inlet channels 201, it exits the catalyst via the outlet channels 202.

The porous material comprising the longitudinal channels 201 and 202 is referred to herein as the “substrate” material. The material that plugs the ends of the channels is typically made from, and coated with, the same substrate material. The substrate material may be any material suitable for internal combustion engine filtering applications, such as cordierite, silicon carbine, aluminum titanate, and metal fiber. This material is referred to herein as “particulate matter filter material”.

The catalytic function of catalyst 11 is achieved by coating the substrate. The inlet channels 201 are coated with a PM catalyst; the outlet channels 202 are coated with an SCR catalyst.

More specifically, the inlet channels 201 are coated with a catalytic material capable of enhancing PM oxidation reactions. Examples of such coatings are coatings containing one or more active elements such as platinum, palladium, rhodium, cerium, zirconium, cobalt, and iron. These coatings are referred to herein as “PM catalyst” coatings. When NO₂ is available, NOx induced passive PM reduction or filter regeneration can occur.

The outlet channels 202 are coated with an SCR catalyst capable of NOx reduction. Catalyst formulations of this type typically contain a zeolite-based catalyst, such as Cu-zeolite SCR, Fe-zeolite SCR, vanadium-based SCR (contains V₂O₅, WO₃, and TiO₂), or any other catalyst with the function of selective reduction of NOx. These coatings are referred to herein as “SCR catalyst” coatings. When a zeolite SCR catalyst is used, PM filter regeneration can be performed at relatively higher temperatures, such as 750 degrees C. and higher.

In operation, exhaust subjected to the coated surface of the inlet channels 201 undergoes reduction and filtering of PM. Exhaust subjected to the coated surface of the outlet channels 202 undergoes selective catalytic reduction (SCR), which converts NOx into nitrogen and water.

With these two types of channels, catalyst 11 can be used as a standalone unit, or as a catalyst component in a multiple unit exhaust aftertreatment system.

Referring again to FIG. 2, a diesel oxidation catalyst (DOC) 29 may optionally be placed upstream of catalyst 11 (relative to the exhaust flow). The DOC 29 converts exhaust nitric oxide (NO) to NO₂ using excess exhaust oxygen. Because catalyst 29 helps to oxidize NO to NO₂, accumulated PM can be oxidized at a lower temperature. Catalyst 29 also increases NOx conversion efficiency of the outlet channels 202. 

1. A wall-flow exhaust gas emissions aftertreatment device for simultaneously reducing the particulate matter and NOx content of the exhaust, comprising: a wall-flow substrate having an entry face and an exit face and a plurality of longitudinal and parallel walls between the entry face and the exit face, the walls defining inlet channels and outlet channels; the substrate being made from a particulate matter filter material; the inlet channels being plugged at the exit face, and the outlet channels being plugged at the entry face; wherein the interiors of the inlet channels are coated with a particulate matter catalyst coating; and wherein the interiors of the outlet channels are coated with a selective catalyst reduction catalyst coating.
 2. The device of claim 1, wherein the substrate is made from one of the following materials: cordierite, silicon carbine, aluminum titanate, or metal fiber.
 3. The device of claim 1, wherein the PM catalyst is made from one or more of the following materials: platinum, palladium, rhodium, cerium, zirconium, cobalt, or iron.
 4. The device of claim 1, wherein the SCR catalyst is a zeolite-based catalyst.
 5. The device of claim 1, wherein the SCR catalyst is made from one of the following materials: Cu-zeolite SCR, Fe-zeolite SCR, or vanadium-based SCR.
 6. The device of claim 1, wherein the number of inlet channels and the number of outlet channels are substantially equal.
 7. The device of claim 1, wherein the respective ends of the inlet channels and outlet channels are plugged in an alternating pattern, such that the entry and exit faces form a checkerboard pattern.
 8. An exhaust gas aftertreatment method for simultaneously reducing the particulate matter and NOx content of the exhaust gas, comprising: directing the exhaust gas to a PM-SCR catalyst having a wall-flow substrate, the substrate having an entry face and an exit face and a plurality of longitudinal and parallel walls between the entry face and the exit face, the walls defining inlet channels and outlet channels; the substrate being made from a particulate matter filter material; the inlet channels being plugged at the exit face, and the outlet channels being plugged at the entry face; wherein the interiors of the inlet channels are coated with a particulate matter catalyst coating; wherein the interiors of the outlet channels are coated with a selective catalyst reduction catalyst coating; and
 8. An exhaust gas aftertreatment method for simultaneously reducing the particulate matter and NOx content of the exhaust gas, comprising: directing the exhaust gas to a PM-SCR catalyst having a wall-flow substrate, the substrate having an entry face and an exit face and a plurality of longitudinal and parallel walls between the entry face and the exit face, the walls defining inlet channels and outlet channels; the substrate being made from a particulate matter filter material; the inlet channels being plugged at the exit face, and the outlet channels being plugged at the entry face; wherein the interiors of the inlet channels are coated with a particulate matter catalyst coating; wherein the interiors of the outlet channels are coated with a selective catalyst reduction catalyst coating; and passing the gas into the entry face such that it travels into the inlet channels, through the coated walls of the inlet channels into the outlet channels, and out of the device via the exit face.
 9. The method of claim 8, further comprising the step of directing the exhaust gas through an oxidation catalyst before directing the exhaust gas to the PM-SCR catalyst.
 10. The method of claim 8, wherein the substrate is made from one of the following materials: cordierite, silicon carbine, aluminum titanate, or metal fiber.
 11. The method of claim 8, wherein the PM catalyst is made from one or more of the following materials: platinum, palladium, rhodium, cerium, zirconium, cobalt, or iron.
 12. The method of claim 8, wherein the SCR catalyst is a zeolite-based catalyst.
 13. The method of claim 8, wherein the SCR catalyst is made from one of the following materials: Cu-zeolite SCR, Fe-zeolite SCR, or vanadium-based SCR.
 14. The method of claim 8, wherein the number of inlet channels and the number of outlet channels are substantially equal.
 15. The method of claim 8, wherein the respective ends of the inlet channels and outlet channels are plugged in an alternating pattern, such that the entry and exit faces form a checkerboard pattern. 